Requirements exist for making very, small accurate distance measurements (approximately 1 to 500 microns) from a reference point that is far away (approximately 1 to 2 meters). In addition, such measurements are typically required to be made when the object being measured is in a vacuum chamber at cryogenic temperatures. The object being measured is under limited access due to its location in the vacuum chamber and usually only an optical path to the object is available. Generally, some type of interferometric optical system is used to measure these small distances.
Interferometric fringe counting systems may be used to measure distances to the object. These fringe counting systems are difficult to set up and maintain, particularly in a vacuum chamber at cryogenic temperatures. These counting systems have very stringent alignment requirements that must be maintained.
Fringe counting by these systems becomes harder if the distances to be measured approach several hundred microns. Vacuum compatible lasers needed for fringe counting systems are expensive and hard to properly operate. Windows in vacuum chambers may cause Etalon effects which make it difficult to move a laser out of the vacuum chamber and still be able to count fringes. Finally, for very accurate distance measurements, an exact wavelength of the laser needs to be known and kept stable. The exactness and stability of the laser wavelength are a function of the metrology error that may be tolerated, but usually are a small fraction of the wavelength.
In general, it takes an elaborate thermal and power control system to maintain the stability of the laser wavelength, and often the laser wavelength must be monitored with an additional measuring system to insure that the stability is maintained. In addition, the optical path must be able to transmit the output of the laser wavelength through narrow band pass optical devices, in which the optical band pass may not encompass an available laser wavelength.
As will be explained, the present invention avoids the complexities associated with fringe counting systems. Any optical beam source may be used by the present invention and a laser source is not necessary. Elaborate band pass filters are not necessary and a simple receiver may be used by the present invention to determine a spot movement, or angle change, in the optical beam to determine a distance movement in the target under observation. The optical path is easy and simple to set up.